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.\"	from: @(#)crash.8	6.5 (Berkeley) 4/20/91
.\"
.Dd $Mdocdate: November 8 2010 $
.Dt CRASH 8
.Os
.Sh NAME
.Nm crash
.Nd system failure and diagnosis
.Sh DESCRIPTION
This section explains what happens when the system crashes
and (very briefly) how to analyze crash dumps.
.Pp
When the system crashes voluntarily it prints a message of the form
.Bd -literal -offset indent
panic: why i gave up the ghost
.Ed
.Pp
on the console and enters the kernel debugger,
.Xr ddb 4 .
.Pp
If you wish to report this panic, you should include the output of
the
.Ic ps
and
.Ic trace
commands.
Unless the
.Sq ddb.log
sysctl has been disabled, anything output to screen will be
appended to the system message buffer, from where it may be
possible to retrieve it through the
.Xr dmesg 8
command after a warm reboot.
If the debugger command
.Ic boot dump
is entered, or if the debugger was not compiled into the kernel, or
the debugger was disabled with
.Xr sysctl 8 ,
then the system dumps the contents of physical memory
onto a mass storage peripheral device.
The particular device used is determined by the
.Sq dumps on
directive in the
.Xr config 8
file used to build the kernel.
.Pp
After the dump has been written, the system then
invokes the automatic reboot procedure as
described in
.Xr reboot 8 .
If auto-reboot is disabled (in a machine dependent way) the system
will simply halt at this point.
.Pp
Upon rebooting, and
unless some unexpected inconsistency is encountered in the state
of the file systems due to hardware or software failure, the system
will copy the previously written dump into
.Pa /var/crash
using
.Xr savecore 8 ,
before resuming multi-user operations.
.Ss Causes of system failure
The system has a large number of internal consistency checks; if one
of these fails, then it will panic with a very short message indicating
which one failed.
In many instances, this will be the name of the routine which detected
the error, or a two-word description of the inconsistency.
A full understanding of most panic messages requires perusal of the
source code for the system.
.Pp
The most common cause of system failures is hardware failure
.Pq e.g., bad memory
which
can reflect itself in different ways.
Here are the messages which are most likely, with some hints as to causes.
Left unstated in all cases is the possibility that a hardware or software
error produced the message in some unexpected way.
.Bl -tag -width indent
.It no init
This panic message indicates filesystem problems, and reboots are likely
to be futile.
Late in the bootstrap procedure, the system was unable to
locate and execute the initialization process,
.Xr init 8 .
The root filesystem is incorrect or has been corrupted, or the mode
or type of
.Pa /sbin/init
forbids execution.
.It trap type %d, code=%x, pc=%x
A unexpected trap has occurred within the system; the trap types are
machine dependent and can be found listed in
.Pa /sys/arch/ARCH/include/trap.h .
.Pp
The code is the referenced address, and the pc is the program counter at the
time of the fault is printed.
Hardware flakiness will sometimes generate this panic, but if the cause
is a kernel bug,
the kernel debugger
.Xr ddb 4
can be used to locate the instruction and subroutine inside the kernel
corresponding
to the PC value.
If that is insufficient to suggest the nature of the problem,
more detailed examination of the system status at the time of the trap
usually can produce an explanation.
.It init died
The system initialization process has exited.
This is bad news, as no new users will then be able to log in.
Rebooting is the only fix, so the system just does it right away.
.It out of mbufs: map full
The network has exhausted its private page map for network buffers.
This usually indicates that buffers are being lost, and rather than
allow the system to slowly degrade, it reboots immediately.
The map may be made larger if necessary.
.El
.Pp
That completes the list of panic types you are likely to see.
.Ss Analyzing a dump
When the system crashes it writes (or at least attempts to write)
an image of memory, including the kernel image, onto the dump device.
On reboot, the kernel image and memory image are separated and preserved in
the directory
.Pa /var/crash .
.Pp
To analyze the kernel and memory images preserved as
.Pa bsd.0
and
.Pa bsd.0.core ,
you should run
.Xr gdb 1 ,
loading in the images with the following commands:
.Bd -literal -offset indent
# gdb
GNU gdb 6.3
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "i386-unknown-openbsd4.6".
(gdb) file /var/crash/bsd.0
Reading symbols from /var/crash/bsd.0...(no debugging symbols found)...done.
(gdb) target kvm /var/crash/bsd.0.core
.Ed
.Pp
[Note that the
.Dq kvm
target is currently only supported by
.Xr gdb 1
on some architectures.]
.Pp
After this, you can use the
.Ic where
command to show trace of procedure calls that led to the crash.
.Pp
For custom-built kernels, it is helpful if you had previously
configured your kernel to include debugging symbols with
.Sq makeoptions DEBUG="-g"
.Pq see Xr options 4
(though you will not be able to boot an unstripped kernel since it uses too
much memory).
In this case, you should use
.Pa bsd.gdb
instead of
.Pa bsd.0 ,
thus allowing
.Xr gdb 1
to show symbolic names for addresses and line numbers from the source.
.Pp
Analyzing saved system images is sometimes called post-mortem debugging.
There are a class of analysis tools designed to work on
both live systems and saved images, most of them are linked with the
.Xr kvm 3
library and share option flags to specify the kernel and memory image.
These tools typically take the following flags:
.Bl -tag -width indent
.It Fl M Ar core
Normally this
.Ar core
is an image produced by
.Xr savecore 8
but it can be
.Pa /dev/mem
too, if you are looking at the live system.
.It Fl N Ar system
Takes a kernel
.Ar system
image as an argument.
This is where the symbolic information is gotten from,
which means the image cannot be stripped.
In some cases, using a
.Pa bsd.gdb
version of the kernel can assist even more.
.El
.Pp
The following commands understand these options:
.Xr fstat 1 ,
.Xr netstat 1 ,
.Xr nfsstat 1 ,
.Xr ps 1 ,
.Xr w 1 ,
.Xr dmesg 8 ,
.Xr iostat 8 ,
.Xr kgmon 8 ,
.Xr pstat 8 ,
.Xr trpt 8 ,
.Xr vmstat 8
and many others.
There are exceptions, however.
For instance,
.Xr ipcs 1
has renamed the
.Fl M
argument to be
.Fl C
instead.
.Pp
Examples of use:
.Bd -literal -offset indent
# ps -N /var/crash/bsd.0 -M /var/crash/bsd.0.core -O paddr
.Ed
.Pp
The
.Fl O Ar paddr
option prints each process'
.Li struct proc
address, but with the value of KERNBASE masked off.
This is very useful information if you are analyzing process contexts in
.Xr gdb 1 .
You need to add back KERNBASE though, that value can be found in
.Pa /usr/include/$ARCH/param.h .
.Bd -literal -offset indent
# vmstat -N /var/crash/bsd.0 -M /var/crash/bsd.0.core -m
.Ed
.Pp
This analyzes memory allocations at the time of the crash.
Perhaps some resource was starving the system?
.Ss Analyzing a live kernel
Like the tools mentioned above,
.Xr gdb 1
can be used to analyze a live system as well.
This can be accomplished by not specifying a crash dump when selecting the
.Dq kvm
target:
.Bd -literal -offset indent
(gdb) target kvm
.Ed
.Pp
It is possible to inspect processes that entered the kernel by
specifying a process'
.Li struct proc
address to the
.Ic kvm proc
command:
.Bd -literal -offset indent
(gdb) kvm proc 0xd69dada0
#0  0xd0355d91 in sleep_finish (sls=0x0, do_sleep=0)
    at ../../../../kern/kern_synch.c:217
217                     mi_switch();
.Ed
.Pp
After this, the
.Ic where
command will show a trace of procedure calls, right back to where the
selected process entered the kernel.
.Sh CRASH LOCATION DETERMINATION
The following example should make it easier for a novice kernel
developer to find out where the kernel crashed.
.Pp
First, in
.Xr ddb 4
find the function that caused the crash.
It is either the function at the top of the traceback or the function
under the call to
.Fn panic
or
.Fn uvm_fault .
.Pp
The point of the crash usually looks something like this "function+0x4711".
.Pp
Find the function in the sources, let's say that the function is in "foo.c".
.Pp
Go to the kernel build directory, e.g.,
.Pa /sys/arch/ARCH/compile/GENERIC .
.Pp
Do the following:
.Bd -literal -offset indent
# rm foo.o
# make DEBUG=-g foo.o
# objdump -S foo.o | less
.Ed
.Pp
Find the function in the output.
The function will look something like this:
.Bd -literal -offset indent
0: 17 47 11 42         foo %x, bar, %y
4: foo bar             allan %kaka
8: XXXX                boink %bloyt
etc.
.Ed
.Pp
The first number is the offset.
Find the offset that you got in the ddb trace
(in this case it's 4711).
.Pp
When reporting data collected in this way, include ~20 lines before and ~10
lines after the offset from the objdump output in the crash report, as well
as the output of
.Xr ddb 4 Ns 's
"show registers" command.
It's important that the output from objdump includes at least two or
three lines of C code.
.Sh REPORTING
If you are sure you have found a reproducible software bug in the kernel,
and need help in further diagnosis, or already have a fix, use
.Xr sendbug 1
to send the developers a detailed description including the entire session
from
.Xr gdb 1 .
.Sh SEE ALSO
.Xr gdb 1 ,
.Xr sendbug 1 ,
.Xr ddb 4 ,
.Xr reboot 8 ,
.Xr savecore 8
